1. Technical Field
The present exemplary embodiments are related to a magnetic resonance diagnostic apparatus and a magnetic resonance diagnostic method which use magnetic resonance phenomenon to obtain an image of a subject.
2. Description of Related Art
In diffusion weighted imaging (DWI), a pair of motion probing gradients (MPG) is added upon imaging. Further, a diffusion weighted image is obtained by imaging a signal difference which appears depending on the extent of phase diffusion of a spin of a proton transferred by diffusion while applying the MPG.
As a value to indicate the impact of an MPG pulse, a b-factor is used. When the b-factor is increased, a contrast due to diffusion becomes stronger, and an image which further reflects a diffusion phenomenon is obtained. This b-factor is expressed by the following equation.
      b    =                            (                      2            ⁢                                                  ⁢            π                    )                2            ⁢                        ∫          0          TE                ⁢                                                            k                →                            ⁡                              (                t                )                                      ·                                          k                →                            ⁡                              (                t                )                                              ⁢                                          ⁢                      ⅆ            t                                                  k        →            ⁡              (        t        )              =                  γ                  2          ⁢                                          ⁢          π                    ⁢                        ∫          0          t                ⁢                                            G              →                        ⁡                          (                              t                ′                            )                                ⁢                                          ⁢                      ⅆ                          t              ′                                          
When an imaging sequence is a widely used single shot SE-EPI (spin echo-echo planar imaging), and in a case where the MPG pulse is applied uniaxially as an ideal Block pulse, the b-factor is expressed by the following equation.b=γ2G2δ2(Δ−δ/3)
Further, G indicates a maximum gradient magnetic field intensity, δ indicates time for applying the MPG pulse, and Δ indicates time difference of the center of two MPG pulse wave forms.
In other words, the b-factor is proportionate to the square of the maximum gradient magnetic field intensity G, or the square of time for applying the MPG pulse.
A thousand or so b-factors widely used in DWI give low diffusion weighted contrast, and a poor portrait of malignant tumor. Therefore, it is desired that the b-factor be increased to improve diffusion weighted contrast. However, if the time δ for applying MPG pulse is increased to increase the b-factor, the TE (echo time) becomes extended, thereby, causing SNR (signal-to-noise ratio) to deteriorate and motion artifact to increase. However, in order to increase maximum gradient magnetic field intensity G to obtain a large b-factor while maintaining a constant TE, it is necessary to improve the hardware performance of the gradient magnetic field system, which requires the cost to increase.
Further, JP-A 2008-12172 (KOKAI) (US 2008/0007264 A1) discloses a technique to generate an image for reading X-rays by processing a DWI image.